φ-Meson production at forward rapidity in p–Pb collisions at √sNN=5.02 TeV and in pp collisions at √s=2.76 TeV

Physics Letters B 768 (2017) 203–217 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb φ-Meson production at forward rapidity in p–Pb collisions at √sNN =5.02 TeV and in pp collisions at √s=2.76 TeV .ALICE Collaboration a r t i c l e i n f o a b s t r a c t Article history: Received 16 November 2015 Received in revised form 8 December 2016 Accepted 9 January 2017 Available online 16 February 2017 Editor: L. Rolandi The first study of φ-meson production in p–Pb collisions at forward and backward rapidity, at a nucleon– nucleon centre-of-mass energy √sNN =5.02 TeV, has been performed with the ALICE apparatus at the LHC. The φ-mesons have been identified in the dimuon decay channel in the transverse momentum (pT) range 1 <pT<7GeV/c, both in the p-going (2.03 <y <3.53) and the Pb-going (−4.46 <y <−2.96) directions — where ystands for the rapidity in the nucleon–nucleon centre-of-mass — the integrated luminosity amounting to 5.01 ±0.19 nb−1and 5.81 ±0.20 nb−1, respectively, for the two data samples. Differential cross sections as a function of transverse momentum and rapidity are presented. The forward–backward ratio for φ-meson production is measured for 2.96 <|y| <3.53, resulting in a ratio ∼0.5with no significant pTdependence within the uncertainties. The pTdependence of the φnuclear modification factor RpPb exhibits an enhancement up to a factor 1.6 at pT=3–4 GeV/cin the Pb-going direction. The pTdependence of the φ-meson cross section in pp collisions at √s=2.76 TeV, which is used to determine a reference for the p–Pb results, is also presented here for 1 <pT<5GeV/cand 2.5 <y <4, for a 78 ±3nb −1integrated luminosity sample. ©2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. 1. Introduction Proton–nucleus (p–A) collisions are of special interest in the context of high-energy nuclear physics for two reasons. On one hand, a precise characterisation of particle production processes in p–A collisions is needed as a reference for nucleus–nucleus data. This allows in-medium effects — linked to the formation of a deconfined phase of the QCD matter, the quark–gluon plasma (QGP) [1–3] — to be disentangled from the effects already present in cold nuclear matter. Among them, a sizeable role is played by the transverse momentum broadening of initial-state partons due to multiple scattering inside the nucleus, responsible for the Cronin effect [4], which may lead to an enhancement of intermediate-pThadron spectra. In addition, p–A collisions at LHC energies provide a way to probe the parton distributions of the colliding nucleus at small values of Bjorken-x, in a regime where parton densities can reach saturation [5,6]. In particular, the smallest xvalues contributing to the wave function of the colliding nucleus can be probed by looking at particle production at large rapidities, in the p-going direction. Such a measurement can thus extend towards lower x-values the results of the lowerenergy measurements by the PHOBOS and BRAHMS experiments at RHIC [7,8]. Measurements of identified particle production may, E-mail address: [email protected]. in particular, provide useful constraints for forthcoming theoretical studies of the saturation mechanism at small x. We have already reported results on charged particle production in p–Pb collisions at mid-rapidity. These results focused on the pseudorapidity density [9] and the pTdependence of the nuclear modification factor [10–12]; the latter was found to be consistent with unity for pT2GeV/c. The nuclear modification factor of charged hadrons was also studied by the BRAHMS and PHOBOS Collaborations in d–Au collisions at the nucleon–nucleon centreof-mass energy √sNN =200 GeV at RHIC [13,14], as a function of pseudorapidity, where values smaller than unity were found for η1, corresponding to the d-going direction. In this Letter we report the measurement of φ-meson production at forward rapidity in p–Pb collisions at √sNN =5.02 TeV in the transverse momentum (pT) range 1 <pT<7GeV/c, for the centre-of-mass rapidity (y) ranges 2.03 <y <3.53 (p-going direction) and −4.46 <y <−2.96 (Pb-going direction), in the dimuon decay channel with the ALICE detector. This measurement extends the investigation of light-flavour particle production to forward rapidity. At the same time, it represents an essential baseline for the understanding of φproduction in heavy-ion collisions, where an enhancement of strange particle yields relative to the ones measured in pp collisions has been proposed long ago as a signature of the formation of a QGP phase [15–17] triggering an intense experimental effort already at SPS and RHIC energies [18–24]. It should http://dx.doi.org/10.1016/j.physletb.2017.01.074 0370-2693/©2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. 204 ALICE Collaboration / Physics Letters B 768 (2017) 203–217 be noted that, despite its hidden strangeness, producing a φ-meson in a hadronic collision still implies the creation of a s¯ spair as it is the case for other strange hadrons, even if the hadronisation mechanisms can differ in reason of the different quark composition. In this context, the p–Pb data presented here will provide an important reference for future measurements in Pb–Pb collisions in the LHC Run 2, which will be performed at a comparable energy. The differential φ-meson cross section as a function of transverse momentum is also presented for pp collisions at √s= 2.76 TeV. This measurement complements the ALICE results on φ-meson production in pp collisions at √s=7TeV, already reported in [25] and, combined with the latter, is used to build the pp reference for the p–Pb measurements presented here. 2. Experimental setup A full description of the ALICE detector can be found in [26,27]. The results presented in this Letter have been obtained detecting muon pairs with the muon spectrometer, covering the pseudorapidity region −4 <ηlab <−2.5. Here and in the following, the sign of ηlab is determined by the choice of the LHC reference system. The other detectors relevant for the analysis are the Silicon Pixel Detector (SPD) of the Inner Tracking System (ITS), the V0 detector and the Zero Degree Calorimeters (ZDC). The elements of the muon spectrometer are a hadron absorber, followed by a set of tracking stations, a dipole magnet, an iron wall acting as muon filter and a set of trigger stations. The hadron absorber is made of carbon, concrete and steel and is placed 0.9 m away from the interaction point. Its total material budget corresponds to 10 hadronic interaction lengths. The dipole magnet provides an integrated magnetic field of 3T·min the vertical direction. The muon tracking is provided by five tracking stations, each one composed of two cathode pad chambers. The first two stations are located upstream of the dipole magnet, the third one in the middle of its gap and the last two downstream of it. A1.2m thick iron wall, corresponding to 7.2 hadronic interaction lengths, is placed between the tracking and trigger detectors and absorbs the residual secondary hadrons emerging from the hadron absorber. The hadron absorber together with the iron wall stops muons with total momentum lower than ∼4GeV/c. The muon trigger detector consists of two stations, each one composed of two planes of resistive plate chambers, installed downstream of the muon filter. The SPD consists of two silicon pixel layers, covering the pseudorapidity regions |ηlab| <2.0 and |ηlab| <1.4for the inner and outer layer, respectively. It is used for the determination of the primary interaction vertex position. The V0 is composed of two scintillator hodoscopes covering the pseudorapidity regions 2.8 < ηlab <5.1 and −3.7 <ηlab <−1.7. It is used in the definition of the minimum bias trigger signal, and allows the offline rejection of beam-halo and beam-gas interactions to be performed. The ZDC detectors, positioned symmetrically at 112.5 m from the interaction point, are used to clean the event sample by removing beam–beam collisions not originating from nominal LHC bunches. 3. Data selection and signal extraction The analysis presented in this Letter is based on two data samples, collected by ALICE during the 2013 p–Pb and pp runs at √sNN =5.02 TeV and √s=2.76 TeV, respectively. In this section we present the details of the data selection, as well as the procedure followed for the extraction of the φ-meson signal. 3.1. Data selection The Minimum-Bias (MB) trigger for the considered data sample is given by the logical AND of the signals in the two V0 detectors [28]. Events containing a muon pair are selected by means of a specific dimuon trigger, based on the detection of two muon candidate tracks in the trigger system of the muon spectrometer, in coincidence with the MB condition. Due to the intrinsic momentum cut imposed by the detector, only muons with pT0.5GeV/c manage to leave a signal in the trigger chambers. Because of the different energy of the LHC proton and Pb beams (Ep=4TeV, EPb =1.58 A ·TeV), in p–Pb collisions the nucleon– nucleon centre-of-mass moves in the laboratory with a rapidity y0=0.465 in the direction of the proton beam. The directions of the proton and Pb beam orbits were inverted during the p–Pb data taking period. This allowed the ALICE muon spectrometer to access two different rapidity regions1: the region 2.03 <y <3.53 where the proton beam is directed towards the muon spectrometer (p-going direction) and the region −4.46 <y <−2.96 where the Pb beam is directed towards the muon spectrometer (Pb-going direction). In the following, these two rapidity ranges are also referred to as “forward” and “backward”, respectively. For pp collisions at √s=2.76 TeV the muon spectrometer covers the rapidity region 2.5 <y <4.2 Background events not coming from beam–beam interactions are rejected by performing an offline selection, based on the requirement that the timing signals from the V0 and ZDC detectors are compatible with a collision occurring in the fiducial interaction region |zvtx| 10 cm. The integrated luminosity for the p–Pb data samples was evaluated as Lint =NMB/σMB, where NMB is the number of MB events corresponding to the analysed triggered events, and σMB the MB trigger cross section. The value of NMB was obtained by averaging the results of two different methods — one based on the ratio of trigger rates and the other based on the offline selection of dimuon events in the MB data sample [29] — while the MB trigger cross sections σMB were measured with a van der Meer scan and found to be 2.09 ±0.07 b and 2.12 ±0.07 b, respectively, for the beam configurations corresponding to the forward and backward rapidity coverage of the muon spectrometer [30]. For the pp data sample, the integrated luminosity is calculated with the method described in [31], using as reference the MB trigger cross section σMB =47.7 ±0.9mb, measured in a van der Meer scan [32]. The resulting values of Lint for the analysed p–Pb data samples are 5.01 ±0.19 nb−1and 5.81 ±0.20 nb−1[29,30] — corresponding to ∼24 000 and ∼26 000 reconstructed φ→μμ decays (see next section) — respectively for the forward and backward rapidity regions. For the pp data sample, the integrated luminosity amounts to 78 ±3nb −1for a total number of ∼1 400 reconstructed φ→μμ decays. Track reconstruction in the muon spectrometer is based on a Kalman filter algorithm [25,33,34]. Muon identification is performed by requiring the candidate track to match a track segment in the trigger chambers (trigger tracklet). This request selects muons with pT,μ0.5GeV/cand, as a consequence, significantly affects the collected statistics for dimuons with invariant mass 1GeV/c2and pT1GeV/c. It is also required that muon tracks lie in the pseudorapidity interval −4 <ημ<−2.5, where ημis defined in the laboratory frame, in order to remove the tracks close to the acceptance borders of the spectrometer, where the acceptance drops abruptly. Selected tracks are finally required to exit the hadron absorber at a radial distance from the beam axis, Rabs, in the range 17.6 <Rabs <89.5cm: this cut, for all practical purposes equivalent to the one on ημ, explicitly ensures the rejection 1The sign of yis defined by assuming the proton beam to have positive rapidity. 2In this case the sign of yis defined by assuming the proton beam entering the muon spectrometer to have positive rapidity. ALICE Collaboration / Physics Letters B 768 (2017) 203–217 205 Fig. 1. Dimuon mass spectrum after combinatorial background subtraction: pT-integrated pp sample (top panels) and pT-integrated p–Pb sample in the backward (centre panels) and forward (bottom panels) rapidity regions, compared to the result of the hadronic-cocktail and the empirical-function fits (leftand right-column panels, respectively). Error boxes on data points (well visible only in some regions on the plots) represent the systematic uncertainty due to the combinatorial background subtraction, while error bars account for the statistical uncertainty. The width of the hadronic-cocktail fit result (red band) combines the statistical uncertainties of the free fit parameters with the systematic uncertainties on the fixed parameters (see text). (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.) of tracks crossing the region of the absorber with the highest density material, where multiple scattering and energy loss effects are large and can affect the mass resolution. Muon pairs are built combining two muon tracks that satisfy the above cuts. 3.2. Signal extraction The Opposite-Sign (OS) muon pairs are composed of correlated and uncorrelated pairs. The former contain the signal of interest for the present analysis, while the latter — mainly coming from semi-muonic decays of pions and kaons — form a combinatorial background. The contribution of the combinatorial background to the OS mass spectrum was evaluated using an event mixing technique in which uncorrelated pairs are formed with muons taken from different events. A detailed description of the technique can be found in [25]. The ratio between correlated and uncorrelated OS dimuons at the φ-meson mass is ∼0.65 (∼0.40) in p–Pb collisions at √sNN =5.02 TeV at forward (backward) rapidity, and ∼1.30 in pp collisions at √s=2.76 TeV. A direct comparison of the raw OS mass spectrum and the associated combinatorial background is presented in [35], for each of the pTintervals considered in the analysis. The invariant mass spectra in pp and p–Pb collisions, obtained after combinatorial background subtraction, are shown in Fig. 1 for the pT-integrated samples. In the left-column panels of Fig. 1, the signal is described in the low-mass region (from the threshold 206 ALICE Collaboration / Physics Letters B 768 (2017) 203–217 up to ∼1.5GeV/c2) by the superposition of a so-called hadronic cocktail and the open charm and open beauty processes. The processes included in the hadronic cocktail are the two-body and Dalitz decays of the light neutral mesons η, ρ, ω, ηand φ, which dominate dimuon production for invariant masses below ∼1GeV/c2. The open charm and open beauty contributions arise from correlated semi-muonic decays of charm and beauty mesons and baryons. The hadronic cocktail was simulated with a dedicated generator described in [25], tuned to the existing measurements whenever possible, otherwise based on the kinematic distributions extracted from PYTHIA [36]. In particular, the kinematic distributions of the φ-meson have been tuned by means of an iterative procedure to the results presented in this Letter to ensure self-consistency for this analysis. The open charm and beauty generation is based on a parameterisation of the spectra generated with PYTHIA [33]. The detector response for all these processes is obtained with a simulation based on the GEANT3 [37] transport code. Simulated events are then subjected to the same reconstruction and selection procedure as the data. When describing the signal with the superposition of the aforementioned contributions, four parameters are adjusted in the fit procedure in each of the pTor rapidity intervals considered in the analysis: the yield of the η, ωand φ-mesons, and the one of the open charm and beauty processes, with the relative beauty/charm contribution fixed (see later in this paragraph). In this way, each parameter is linked to a process dominating in at least one region of the considered mass spectrum. The remaining degrees of freedom are fixed either according to the relative branching ratios known from literature [38], or assuming specific hypotheses on the cross section ratios. In particular, the production cross section of the ρ-meson is assumed to be the same as for the ωas suggested from both models and pp data [25], while the ηcontribution was derived from the ηcross section by applying the ratio of the corresponding cross sections ση/ση=0.3taken from the PYTHIA tunes ATLAS-CSC and D6T which best describe the available lowmass dimuon measurements at the LHC energies [25]. The open beauty normalisation is fixed to the open charm one via a fit of the pTand rapidity-integrated mass spectra in which the yields from both processes are free parameters; when performing differential studies, the beauty/charm ratio is scaled according to the differential distributions for the two processes, given by the Monte Carlo (MC) simulations. For each pTand rapidity interval, the raw number of φ-mesons is determined via a fit procedure based on a χ2minimisation, performed on the signal obtained after the subtraction of the combinatorial background, shown in Fig. 1 for the pT-integrated samples. Several tests have been performed to evaluate the robustness of the signal extraction and estimate an appropriate systematic uncertainty for it. They include in particular: –Replacing the fit based on the full MC hadronic cocktail with a fit based on the superposition of various empirical functions. In this case, illustrated in the right-column panels of Fig. 1, the continuum is modelled either with exponential functions or variable-width Gaussians, while the ρ+ωand φpeaks are described by Crystal Ball functions [39] tuned on the MC. – Varying the ratio between the yields of open beauty and open charm processes. It was verified that for perturbations as large as ±50% (resulting in a reasonably wide range of variation for the shape of the total continuum) no significant systematic effect is visible. – Varying the ratios between the two-body and Dalitz branching ratios of the ηand ω-mesons, as well as the cross section ratios σρ/σωand ση/ση, within the uncertainties coming either from the available measurements or from the differences between the PYTHIA tunes considered in the analysis of the pp data. The branching ratio BRω→μμ was taken as the average (weighted by the corresponding uncertainties) of the available measurements of BRω→μμ and BRω→ee [38], assuming lepton universality. – Varying the considered fit range: in particular, the fit was performed both including and excluding the mass region from 0.4 to 0.65 GeV/c2where the quality of the comparison between the data and the sum of the MC sources turns out to be lower. The total systematic uncertainty on the signal extraction was taken as the quadratic sum of the above sources. The systematic uncertainty on the combinatorial background is estimated by comparing the shape of the Like-Sign dimuon contributions coming from the event mixing procedure and from the raw data [25]. This uncertainty depends on the mass, its relative contribution being maximal in the mass window 0.5–0.8GeV/c2and minimal around the φ-meson peak, and it is added in quadrature, for each point of the mass spectrum, to the statistical uncertainty of the signal: in this way, this source of systematics is accounted for by the χ2minimisation procedure, and automatically propagated when evaluating the φ-meson raw signal from the fit parameters. The uncertainty associated to the sum of the MC sources (red band in the leftcolumn plots of Fig. 1) is evaluated by combining the uncertainties on the normalisation of each considered process. For the processes whose normalisation is left free in the fit, this uncertainty is the statistical one resulting from the fit procedure itself; for the rest of the processes, we also propagate the systematic uncertainty on the parameters (branching ratios or cross section ratios) which fix their normalisations to those of the free processes. 4. Results The results of the φ-meson analysis are presented as follows. We first present the measurement of the production cross sections, starting with its pT-dependence in pp collisions at √s=2.76 TeV, followed by p–Pb collision results as a function of pTand rapidity. Then, we show the ratio of the cross sections measured in the forward and backward regions, obtained in the common rapidity interval 2.96 <|y| <3.53. Finally, the measurement of the nuclear modification factor RpPb as a function of pTis presented, separately for the p-going and the Pb-going directions. 4.1. Production cross section in pp and p–Pb collisions The cross section σφwas evaluated for each pTand rapidity interval as: σφ(x)= Nraw φ→μμ(x) [A·ε](x)·BRφ→μμ ·Lint , where xstands for any specific pTor rapidity interval considered. The total systematic uncertainty on Nraw φ→μμ(x), after combining the different sources described above, ranges between 3% and 8% depending on the collision system and kinematic range. The branching ratio BRφ→μμ was taken from [38] as the average (weighted by the corresponding uncertainties) of the available measurements of BRφ→μμ and BRφ→ee, assuming lepton universality, resulting in a final uncertainty of approximately 1%. The product of the geometrical acceptance Aand the reconstruction efficiency εhas been evaluated by means of MC simulations, using the cocktail predictions for the differential input spectra. The values are obtained as the ratio between the number of dimuons at the output of the reconstruction chain — including the effect of the event selection ALICE Collaboration / Physics Letters B 768 (2017) 203–217 207 Fig. 2. φ-meson cross section as a function of pTin pp collisions at √s=2.76 TeV. Error bars and boxes represent statistical and systematic uncertainties, respectively. Predictions from PHOJET [42] and the PYTHIA tunes ATLAS-CSC [44],D6T [45], Perugia0 and Perugia11 [43] are also shown for comparison, as well as the result of a fit with the Levy–Tsallis function defined by Eq. (1). criteria imposed on the data — and the number of dimuons injected as input. The uncertainty on [A ·ε]mainly originates from the systematic uncertainty on the dimuon tracking and trigger efficiencies. The systematic uncertainty on the tracking efficiency, amounting to 6% and 4% for the backward and forward rapidity regions, respectively, comes from the residual differences between the results of the efficiency-determination method based on reconstructed tracks [29,40], applied to both data and MC. For the systematic uncertainty on the trigger efficiency, we also refer to the procedure discussed in [29], resulting in an uncertainty of 3.2% and 2.8%, respectively, for the backward and forward rapidity regions considered in the analysis. In order to test possible additional systematic effects related to the hardware trigger pTcut, imposing a non-sharp threshold around 0.5 GeV/c, the analysis was repeated with the additional offline sharp cuts pT,μ>0.5GeV/cand pT,μ>1GeV/con single muons. For each of the two alternative scenarios, the corresponding measurement of the φ-meson cross section was compared to the one coming from the reference analysis: the difference between the results was found to be smaller than the quadratic difference of the statistical uncertainties, showing that no significant bias related to the trigger threshold affects the results [41]. The reported values correspond to a zero-polarisation scenario for the 2-body decay of the φ-meson, in the absence of evidence supporting less trivial assumptions (in particular, no measurement of φ-meson polarisation is currently available at the LHC energies). 4.1.1. Production cross section in pp collisions The inclusive, pT-differential φ-meson cross section in pp collisions at √s=2.76 TeV is shown in Fig. 2. The data points, also summarised in Table 1, are compared with the predictions from PHOJET [42] and PYTHIA [36], where for the latter the Perugia0,Perugia11 [43],ATLAS-CSC [44], and D6T [45] tunes are considered. An overall good agreement is found between predictions and data, with the exception of the Perugia0 and Perugia11 tunes of PYTHIA which underestimate the measured cross section by a factor of two, as already observed for the φ-meson measurements at √s=7TeV[25,46]. It is worth to note that the D6T tune is not successful in describing the pTevolution of the K/πratio at mid-rapidity in pp collisions at √s=2.76 TeV, as measured by the CMS Collaboration [47]: this suggests that hidden strangeness is better reproduced than open strangeness in this Table 1 pT-differential production cross section for the φ-meson in pp collisions at √s= 2.76 TeV, for 2.5 <y <4. The first uncertainty is statistical and the second is the bin-to-bin uncorrelated systematic. The bin-to-bin correlated relative systematic uncertainty is 3.9%. The χ2/ndf values are relative to the hadronic-cocktail fit and the [0.8, 1.2GeV/c2]mass region, where ndf =10. pT(GeV/c)χ2/ndf d2σφ/(dydpT)(mb/(GeV/c)) [1.0,1.5]1.1 0.423 ±0.067 ±0.043 [1.5,2.0]1.7 0.182 ±0.025 ±0.018 [2.0,2.5]1.1 0.089 ±0.011 ±0.007 [2.5,3.0]1.1 0.0340 ±0.0056 ±0.0020 [3.0,3.5]0.9 0.0139 ±0.0032 ±0.0011 [3.5,4.0]1.1 0.0087 ±0.0022 ±0.0006 [4.0,5.0]1.1 0.0028 ±0.0012 ±0.0002 Table 2 Systematic uncertainties (in percent) contributing to the measurement of the φcross section in pp collisions at √s= 2.76 TeV. When the uncertainty values depend on the pTinterval, their minimum and maximum values are quoted. Source Syst. uncertainty on σpp φ Uncorrelated Signal extraction 3–8% Tracking efficiency 4% Trigger efficiency 3% Correlated Lint 3.8% BR(φ →) 1% specific PYTHIA tune. Data points were fitted with a Levy–Tsallis function [48] 1 pT dN dpT∝1+mT−mφ nT −n ,(1) where mT=p2 T+m2 φstands for the transverse mass, obtaining the values n =10.2 ±4.8 and T=284 ±72 MeV for the fit parameters, where the errors reflect the statistical uncertainties only. The cross section integrated over the accessible pTrange 1 <pT<5GeV/cis σφ=0.566 ±0.055 (stat.) ±0.044 (syst.) mb. The systematic uncertainties for this measurement are summarised in Table 2. 4.1.2. Production cross section in p–Pb collisions The φcross section as a function of pTin p–Pb collisions is shown in Fig. 3 for the forward and backward rapidity regions considered in the analysis. The results, also reported in Table 3, are fitted with the Levy–Tsallis distribution defined in Eq. (1), the resulting fit parameters being β=9.6 ±1.3 and T=366 ±30 MeV for the forward rapidity region and β=11.4 ±1.4 and T=384 ± 24 MeV for the backward one, where the errors reflect the statistical uncertainties only. The predictions from HIJING (with gluon shadowing) [49] and DPMJET [50] are also shown: these generators provided a good description of the ALICE dNch/dηlab results at mid-rapidity [9]. Averaging over the available pTrange, the discrepancy between the data and the predictions from HIJING and DPMJET amounts to ∼18% and ∼57%, respectively, at backward rapidity (the Pb-going direction) and ∼5% and ∼9.5%, respectively, at forward rapidity (the p-going direction). In all the cases, the generators underestimate the data points. The φcross section in p–Pb collisions, integrated over the accessible pTrange, 1 <pT<7GeV/c, is shown as a function of rapidity in Fig. 4. The data points, also summarised in Table 4, exhibit a significant asymmetry between the forward and backward rapidity regions. The data point from the φ-meson analysis at mid-rapidity in the K+K−channel [51], also shown for the 208 ALICE Collaboration / Physics Letters B 768 (2017) 203–217 Fig. 3. φ-meson cross section in p–Pb collisions at √sNN =5.02 TeV as a function of pTin the backward (left) and forward (right) rapidity regions. Error bars (smaller than the markers) and boxes represent statistical and systematic uncertainties, respectively. Predictions by HIJING [49] and DPMJET [50] are also shown, together with the result of a fit with the Levy–Tsallis function (Eq. (1)). Table 3 Production cross section for the φ-meson in p–Pb collisions at √sNN =5.02 TeV, as a function of pT, in the backward and forward rapidity regions. The first uncertainty is statistical and the second is the bin-to-bin uncorrelated systematic. The bin-to-bin correlated relative systematic uncertainty is 3.6% and 3.9%, respectively, for the backward and the forward regions. The χ2/ndf values are relative to the hadronic-cocktail fit and the [0.8, 1.2GeV/c2]mass region. pT(GeV/c)−4.46 <y<−2.96 2.03 <y<3.53 χ2/ndf d2σpPb φ/(dydpT)(mb/(GeV/c)) χ2/ndf d2σpPb φ/(dydpT)(mb/(GeV/c)) [1.0,1.5]0.7 102 ±8±12 1.5 73.3±5.6±8.0 [1.5,2.0]1.2 58.6±3.3±5.51.942.1±2.5±4.3 [2.0,2.5]2.5 28.3±1.4±2.91.721.0±1.2±2.0 [2.5,3.0]4.2 15.0±0.7±1.23.110.07 ±0.77 ±0.97 [3.0,3.5]2.6 7.66 ±0.40 ±0.70 2.0 6.38 ±0.41 ±0.61 [3.5,4.0]1.9 4.20 ±0.24 ±0.34 1.2 3.96 ±0.30 ±0.36 [4.0,4.5]0.7 2.15 ±0.17 ±0.16 1.0 1.99 ±0.20 ±0.15 [4.5,5.0]0.9 1.20 ±0.11 ±0.10 0.9 1.06 ±0.13 ±0.08 [5.0,6.0]1.0 0.560 ±0.052 ±0.054 1.0 0.570 ±0.088 ±0.043 [6.0,7.0]1.2 0.201 ±0.030 ±0.028 0.9 0.199 ±0.045 ±0.016 Fig. 4. φcross section in p–Pb collisions at √sNN =5.02 TeV as a function of rapidity, integrated over the range 1 <pT<7GeV/c. Error bars and boxes represent statistical and systematic uncertainties, respectively. Predictions by HIJING and DPMJET are also shown, together with the mid-rapidity data point from the φ-meson measurement in the K+K−channel [51], also evaluated in the range 1 <pT<7GeV/c. 1 <pT<7GeV/cp Trange, fits well into the trend defined by the two series of points in the backward and forward rapidity regions. This observation complements the previous measurements of light-flavour particle production (charged unidentified particles) reported in p–Pb by ALICE at the LHC at mid-rapidity [9], and in d–Au by PHOBOS at RHIC ranging from mid to forward rapidity [14]. The comparison between the data and the predictions by Table 4 Production cross section for the φ-meson in p–Pb collisions at √sNN =5.02 TeV, as a function of rapidity, integrated over the range 1 <pT<7GeV/c. The first uncertainty is statistical and the second is the bin-to-bin uncorrelated systematic. The bin-to-bin correlated relative systematic uncertainty is 3.6% and 3.9%, respectively, for the backward and the forward regions. The χ2/ndf values are relative to the hadronic-cocktail fit and the [0.8, 1.2GeV/c2]mass region. yχ2/ndf dσpPb φ/dy(mb) yχ2/ndf dσpPb φ/dy(mb) [−4.46,−4.25]0.9 89 ±10 ±9[2.03,2.35]2.6 104 ±11 ±6 [−4.25,−4.05]1.8 89 ±6±7[2.35,2.55]1.5 102 ±7±5 [−4.05,−3.85]0.9 103 ±5±8[2.55,2.75]2.0 96 ±5±6 [−3.85,−3.65]2.9 117 ±6±9[2.75,2.95]1.6 86 ±4±5 [−3.65,−3.45]1.2 128 ±7±9[2.95,3.15]2.3 68 ±4±4 [−3.45,−3.25]3.6 133 ±9±9[3.15,3.35]1.0 66 ±5±5 [−3.25,−2.96]1.2 136 ±14 ±11 [3.35,3.53]1.2 45 ±8±6 HIJING and DPMJET, illustrated in Fig. 4, clearly shows how the models — which successfully described charged particle production at mid-rapidity in the same collision system [9] — fail to properly reproduce the shape and the normalisation of the observed rapidity dependence of the φcross section. Still, the HIJING prediction qualitatively reproduces the forward–backward asymmetry observed in the data, as well as — ignoring the normalisation — the shape of the y-dependence in the backward region. DPMJET, on the contrary, fails to reproduce even qualitatively the observed forward–backward asymmetry. 4.2. Forward–backward ratio in p–Pb collisions To establish a more direct comparison of the cross section in the p-going and Pb-going directions, σpPb φwas extracted as a func- ALICE Collaboration / Physics Letters B 768 (2017) 203–217 209 Fig. 5. Forward–backward ratio for the φ-meson in p–Pb collisions at √sNN = 5.02 TeV as a function of pT, in the rapidity range 2.96 <|y| <3.53 common to the two rapidity regions considered in the analysis. Error bars and boxes represent statistical and systematic uncertainties, respectively. The blue box on the left represents the bin-to-bin correlated systematic uncertainty, see Table 7. Predictions from HIJING and DPMJET are also shown for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) tion of pTin the common |y|range 2.96 <|y| <3.53. The pT interval 1.0 <pT<1.5GeV/cwas discarded in this measurement because of the poor statistics available in this limited rapidity range, resulting in an uncertainty larger than 50%. The ratio between the forward and backward cross section, RFB, is shown as a function of pTin Fig. 5. The data points exhibit no significant pTdependence within the experimental uncertainties. Predictions by HIJING and DPMJET are also shown, with HIJING slightly overestimating the data points and DPMJET clearly failing to reproduce the observed values, staying above RFB =1in the whole pTrange considered here. This observation is consistent with the observations in Fig. 4, where the forward– backward asymmetry of the φ-meson yield was better reproduced by HIJING than by DPMJET. 4.3. Nuclear modification factor in p–Pb collisions The φ-meson nuclear modification factor RpPb is defined as the ratio between the production cross section σpPb φ(pT)in p–Pb collisions and the cross section σpp φ(pT)in pp collisions — evaluated at √s=5.02 TeV as described in the following — scaled by APb: RpPb(pT)=σpPb φ(pT) σpp φ(pT)·APb ,(2) where APb is the nuclear mass number for the Pb nucleus. Since for the pp cross section σpp φat √s=5.02 TeV no direct measurement is currently available, it was evaluated by interpolating the measurements in the rapidity interval 2.5 <y <4at √s=2.76 (see Section 4.1.1) and 7TeV[25]. For each pTinterval, the √s dependence of the differential cross section d2σpp φ/(dy dpT)was described with a power law σpp(√s) =C·(√s)α, where Cand α are determined using the data at 2.76 and 7TeV. Alternative parameterisations were also considered [52], namely a linear and an exponential function, and the mean of the results obtained with the three functions was taken. Since the pp measurements are limited to 1 <pT<5GeV/c, the cross section at √sNN =5.02 TeV was extrapolated towards higher pTby means of a Levy–Tsallis function, which describes the calculated differential cross section in the pTrange covered by the measurements. The uncertainty Table 5 Differential cross section for the φ-meson in pp collisions at √s=5.02 TeV in the backward and forward rapidity regions of interest for the analysis of the p–Pb data, as obtained interpolating the available measurements at √s=2.76 and 7TeV. Total uncertainties, combining statistical and systematic sources, are reported. pT(GeV/c)d 2σφ/dydpT(mb/(GeV/c)) −4.46 <y<−2.96 2.03 <y<3.53 [1.0,1.5]0.491 ±0.067 0.656 ±0.090 [1.5,2.0]0.223 ±0.015 0.297 ±0.020 [2.0,2.5]0.0995 ±0.0071 0.1328 ±0.0095 [2.5,3.0]0.0467 ±0.0032 0.0623 ±0.0043 [3.0,3.5]0.0234 ±0.0015 0.0312 ±0.0020 [3.5,4.0]0.0125 ±0.0011 0.0167 ±0.0015 [4.0,4.5]0.00706 ±0.00094 0.0094 ±0.0012 [4.5,5.0]0.00419 ±0.00082 0.0056 ±0.0011 [5.0,6.0]0.00213 ±0.00060 0.00284 ±0.00081 [6.0,7.0]0.00093 ±0.00039 0.00124 ±0.00052 on the interpolated cross sections arises from the choice of the function used for the interpolation, from the uncertainties in the measurements at 2.76 and 7TeV, and — for pT>5GeV/c— from the extrapolation based on the Levy–Tsallis fit. They range from about 7% for pT=1GeV/cto 20% for pT=5GeV/c, and exceed 30% for pT>5GeV/c, representing the major source of systematic uncertainty for the measurement of the nuclear modification factor. The interpolated cross section, which refers to the rapidity range 2.5 <y <4, was finally scaled to the forward and backward rapidity windows 2.03 <y <3.53 and −4.46 <y <−2.96, considered for the analysis of the p–Pb data. The relative scaling factors ffwd =1.135 ±0.031 and fbkw =0.850 ±0.028 were evaluated as an average from simulations with PHOJET and the Perugia0, Perugia11,ATLAS-CSC and D6T PYTHIA tunes. In doing so, we also retained the PYTHIA tunes which were observed to fail in describing the pp data (see Section 4.1.1): the reason is that the disagreement between models and data concerns in this case the absolute normalisation more than the shape of the kinematic distributions, which is the only relevant feature in the evaluation of the ffwd and fbkw factors. The uncertainties (amounting to about 3%) correspond to the differences between the considered MC predictions. The numerical values are reported in Table 5. The nuclear modification factor RpPb as a function of pTis shown in the two panels of Fig. 6 for the backward and forward rapidity regions considered in the analysis. The numerical values are also quoted in Table 6. For each pTinterval, the systematic uncertainty detailed in Table 7 results from the quadratic sum of the uncertainty on the φcross section in p–Pb and the one of the pp reference. A rising trend of RpPb when going from pT=1GeV/c to pT≈3–4 GeV/ccan be observed both at backward and forward rapidity. The values of RpPb in the two rapidity ranges, however, are significantly different. In particular, at backward rapidity we observe an enhancement of the φcross section with respect to the scaled pp reference peaked around pT=3–4 GeV/c. This enhancement, absent in the forward rapidity region, reaches a factor of up to ∼1.6 and could be associated either to an initial-state effect (including a possible Cronin-like enhancement [4,53]) or to a final state effect related to radial flow in p–Pb as proposed for recent ALICE measurements at mid-rapidity [12]. Discriminating between these two effects requires more detailed investigations, including differential analyses as a function of global event properties like collision centrality. Concerning the behaviour at high pT, we observe that the φ-meson RpPb is compatible with unity for pT4GeV/cin the p-going direction, similar to what was observed for the RpPb of charged particle production at mid-rapidity [10,12]. The observations in the Pb-going direction do not allow a clear trend of the RpPb factor at high pTto be established. A possible saturation at 210 ALICE Collaboration / Physics Letters B 768 (2017) 203–217 Fig. 6. Nuclear modification factor RpPb in p–Pb collisions at √sNN =5.02 TeV for the φ-meson as a function of pT, in the backward (left) and forward (right) rapidity regions considered in the analysis. Error bars and boxes represent statistical and bin-to-bin uncorrelated systematic uncertainties, respectively. The blue box on the left represents the bin-to-bin correlated systematic uncertainty, see Table 7. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 6 Nuclear modification factor RpPb in p–Pb collisions at √sNN =5.02 TeV for the φ-meson as a function of pTin the backward and forward rapidity regions. The first uncertainty is statistical and the second is the bin-to-bin uncorrelated systematic. The bin-to-bin correlated relative systematic uncertainty is 8%. pT(GeV/c)RpPb −4.46 <y<−2.96 2.03 <y<3.53 [1.0,1.5]1.00 ±0.08 ±0.18 0.537 ±0.041 ±0.094 [1.5,2.0]1.26 ±0.07 ±0.15 0.681 ±0.040 ±0.083 [2.0,2.5]1.37 ±0.07 ±0.17 0.760 ±0.043 ±0.091 [2.5,3.0]1.54 ±0.07 ±0.16 0.777 ±0.059 ±0.092 [3.0,3.5]1.57 ±0.08 ±0.18 0.98 ±0.06 ±0.11 [3.5,4.0]1.62 ±0.09 ±0.19 1.14 ±0.09 ±0.15 [4.0,4.5]1.46 ±0.12 ±0.22 1.02 ±0.10 ±0.15 [4.5,5.0]1.38 ±0.13 ±0.29 0.91 ±0.11 ±0.19 [5.0,6.0]1.26 ±0.12 ±0.38 0.97 ±0.15 ±0.29 [6.0,7.0]1.04 ±0.16 ±0.46 0.77 ±0.17 ±0.33 Table 7 Systematic uncertainties (in percent) contributing to the measurement of the φ cross section and nuclear modification factor in the backward and forward rapidity regions in p–Pb collisions at √sNN =5.02 TeV. When the uncertainty values depend on the pTinterval, their minimum and maximum values are quoted. Source Syst. uncertainty on σpPb φand RpPb −4.46 <y<−2.96 2.03 <y<3.53 Uncorrelated Signal extraction 3–5% 4–8% Tracking efficiency 6% 4% Trigger efficiency 3.2% 2.8% σpp φ7–30% 7–30% Correlated Lint 3.5% 3.8% BR(φ →) 1% 1% fbkw 3.3% – ffwd –2.7% RpPb ≈1for pT5GeV/cis, however, still compatible with the measurements. Only few other existing measurements can be compared to our data. In particular, results on φ-meson production in d–Au collisions at √sNN =200 GeV have been recently released by the PHENIX Collaboration [54]. The pT-dependence of the RdAu measured by PHENIX, as well as its evolution from backward to forward rapidity, is found to be similar to what is observed in our results for RpPb. Mid-rapidity data on RdAu, also presented by the PHENIX Collaboration for the φ-meson, seem to sit between the forwardand backward-rapidity results. Forward-rapidity measurements in d–Au collisions at √sNN =200 GeV at RHIC [13,14] are also available for unidentified charged particles, although for the d-going direction only. These data exhibit, similar to our φ-meson results in the p-going direction, a rise of RdAu from ∼0.5to ∼1 between pT∼1GeV/cand pT∼4GeV/c. A similar rise of RpPb in p–Pb collisions at √sNN =5.02 TeV is also observed in the already cited measurement of unidentified charged particle and identified charged pion and kaon production at mid-rapidity performed by ALICE [10,12]. A recent study of φ-meson production in p–Pb collisions at mid-rapidity by ALICE [51] does not currently include results on RpPb. 5. Conclusions We have presented results on φ-meson production in the dimuon channel in p–Pb collisions at √sNN =5.02 TeV obtained by the ALICE experiment at the LHC. Cross section and nuclear modification factor measurements were performed for 1 <pT<7GeV/c in the rapidity windows 2.03 <y <3.53 (p-going direction) and −4.46 <y <−2.96 (Pb-going direction). A corresponding cross section measurement in pp collisions at √s=2.76 TeV has also been reported, for 1 <pT<5GeV/cin the region 2.5 <y <4. Predictions from HIJING and DPMJET are compared to the p–Pb cross sections and are found to underestimate the data both at backward (by about 18% and 57% on average, respectively) and at forward rapidity (by about 5% and 9.5% on average, respectively). The forward–backward ratio in the φ-meson cross section in p–Pb collisions was measured in the rapidity range 2.96 <|y| <3.53, and no significant pTdependence was found within uncertainties. In this case, the data points are significantly overestimated by the DPMJET model, while only a slight disagreement is observed with respect to the HIJING prediction. In the p-going direction a rising trend of the nuclear modification factor RpPb is observed from ∼0.5to ∼1, when going from pT=1GeV/cto pT=4GeV/c. This observation is compatible with the behaviour of charged particles at forward rapidity at RHIC energies, and at mid-rapidity at LHC energies. In the Pbgoing direction, on the other hand, an enhancement is observed for RpPb, reaching values as large as ∼1.6around pT=3–4 GeV/c. An interpretation of these results, either in terms of an initialstate (Cronin-like) effect or a final-state effect related to radial flow in p–Pb, is not possible yet, due to a general lack of theoretical predictions for particle production in the light-flavour sector at forward rapidity in p–A collisions at the LHC energies. ALICE Collaboration / Physics Letters B 768 (2017) 203–217 211 Acknowledgements The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) Collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL), State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences and Nationalstiftung für Forschung, Technologie und Entwicklung, Austria; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Financiadora de Estudos e Projetos (Finep) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Science and Technology of the People’s Republic of China (MOST), National Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOE), China; Ministry of Science, Education and Sports and Croatian Science Foundation, Croatia; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; The Danish Council for Independent Research –Natural Sciences, the Carlsberg Foundation and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l’Energie Atomique (CEA) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany; Ministry of Education, Research and Religious Affairs, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy, Government of India (DAE) and Council of Scientific and Industrial Research (CSIR), New Delhi, India; Indonesian Institute of Science, Indonesia; Centro Fermi – Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi and Instituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Science and Technology, Nagasaki Institute of Applied Science (IIST), Japan Society for the Promotion of Science (JSPS) KAKENHI and Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Academico (DGAPA), Mexico; Nationaal instituut voor subatomaire fysica (Nikhef), Netherlands; The Research Council of Norway, Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru; Ministry of Science and Higher Education and National Science Centre, Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research, Institute of Atomic Physics and Romanian National Agency for Science, Technology and Innovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation and National Research Centre Kurchatov Institute, Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic, Slovakia; National Research Foundation of South Africa, South Africa; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba, Ministerio de Ciencia e Innovación and Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Spain; Swedish Research Council (VR) and Knut and Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research, Switzerland; National Science and Technology Development Agency (NSDTA), Suranaree University of Technology (SUT) and Office of the Higher Education Commission under NRU project of Thailand, Thailand; Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP), United States of America. References [1] A. Bazavov, T. Bhattacharya, M. Cheng, C. DeTar, H. Ding, et al., The chiral and deconfinement aspects of the QCD transition, Phys. Rev. D 85 (2012) 054503, arXiv:1111.1710 [hep-lat]. [2] Wuppertal–Budapest Collaboration, S. Borsanyi, et al., Is there still any Tcmystery in lattice QCD? Results with physical masses in the continuum limit III, J. High Energy Phys. 1009 (2010) 073, arXiv:1005.3508 [hep-lat]. [3] S. Borsanyi, G. Endrodi, Z. Fodor, A. Jakovac, S.D. Katz, et al., The QCD equation of state with dynamical quarks, J. High Energy Phys. 1011 (2010) 077, arXiv: 1007.2580 [hep-lat]. [4] A. Accardi, Cronin effect in proton nucleus collisions: a survey of theoretical models, arXiv:hep-ph/0212148. [5] C. Salgado, J. Alvarez-Muniz, F. Arleo, N. Armesto, M. Botje, et al., Proton– nucleus collisions at the LHC: scientific opportunities and requirements, J. Phys. G 39 (2012) 015010, arXiv:1105.3919 [hep-ph]. [6] M. Brandt, M. Klasen, F. König, Nuclear parton density modifications from lowmass lepton pair production at the LHC, Nucl. Phys. A 927 (2014) 78–90, arXiv: 1401.6817 [hep-ph]. [7] BRAHMS Collaboration, I. Arsene, et al., Quark gluon plasma and color glass condensate at RHIC? The perspective from the BRAHMS experiment, Nucl. Phys. A 757 (2005) 1–27, arXiv:nucl-ex/0410020. [8] B. Back, M. Baker, M. Ballintijn, D. Barton, B. Becker, et al., The PHOBOS perspective on discoveries at RHIC, Nucl. Phys. A 757 (2005) 28–101, arXiv:nuclex/0410022. [9] ALICE Collaboration, B. Abelev, et al., Pseudorapidity density of charged particles p–Pb collisions at √sNN =5.02 TeV, Phys. Rev. Lett. 110 (2013) 032301, arXiv:1210.3615 [nucl-ex]. [10] ALICE Collaboration, B. Abelev, et al., Transverse momentum distribution and nuclear modification factor of charged particles in p–Pb collisions at √sNN = 5.02 TeV, Phys. Rev. Lett. 110 (2013) 082302, arXiv:1210.4520 [nucl-ex]. [11] ALICE Collaboration, B. Abelev, et al., Transverse momentum dependence of inclusive primary charged-particle production in p–Pb collisions at √sNN = 5.02 TeV, Eur. Phys. J. C 74 (9) (2014) 3054, arXiv:1405.2737 [nucl-ex]. [12] ALICE Collaboration, J. Adam, et al., Multiplicity dependence of charged pion, kaon, and (anti)proton production at large transverse momentum in p–Pb collisions at √sNN =5.02 TeV, Phys. Lett. B 760 (2016) 720–735, arXiv:1601.03658 [nucl-ex]. [13] BRAHMS Collaboration, I. Arsene, et al., On the evolution of the nuclear modification factors with rapidity and centrality in d+Au collisions at √sNN = 200 GeV, Phys. Rev. Lett. 93 (2004) 242303, arXiv:nucl-ex/0403005. [14] PHOBOS Collaboration, B. Back, et al., Pseudorapidity distribution of charged particles in d+Au collisions at √sNN =200 GeV, Phys. Rev. Lett. 93 (2004) 082301, arXiv:nucl-ex/0311009. [15] J. Rafelski, B. Muller, Strangeness production in the quark–gluon plasma, Phys. Rev. Lett. 48 (1982) 1066. [16] A. Shor, φ-Meson production as a probe of the quark gluon plasma, Phys. Rev. Lett. 54 (1985) 1122–1125. [17] P. Koch, B. Muller, J. Rafelski, Strangeness in relativistic heavy ion collisions, Phys. Rep. 142 (1986) 167–262. [18] NA49 Collaboration, C. Alt, et al., System-size dependence of strangeness production in nucleus–nucleus collisions at √sNN =17.3GeV, Phys. Rev. Lett. 94 (2005) 052301, arXiv:nucl-ex/0406031. [19] NA49 Collaboration, C. Alt, et al., Energy dependence of φmeson production in central Pb+Pb collisions at √sNN =6to 17 GeV, Phys. Rev. C 78 (2008) 044907, arXiv:0806.1937 [nucl-ex]. [20] NA50 Collaboration, B. Alessandro, et al., φproduction in Pb–Pb collisions at 158-GeV/cper nucleon incident momentum, Phys. Lett. B 555 (2003) 147–155; NA50 Collaboration, B. Alessandro, et al., Phys. Lett. B 561 (2003) 294 (Erratum). [21] NA60 Collaboration, K. Banicz, et al., φproduction in In–In collisions at 158-AGeV, Eur. Phys. J. C 64 (2009) 1–18, arXiv:0906.1102 [hep-ex]. [22] NA60 Collaboration, R. Arnaldi, et al., A comparative measurement of φ→ K+K−and φ→μ+μ−in In–In collisions at the CERN SPS, Phys. Lett. B 699 (2011) 325–329, arXiv:1104.4060 [nucl-ex].